1. Field of the Invention
The present invention relates to a method of producing channeled plastic pipe having a number of air channels continuously axially extending through the wall thereof and more particularly, to such a pipe wherein at least one of the respective ends of said air channels are closed and the inner and outer surfaces of the closed region are finished as smooth surface.
2. Description of the Prior Art
There has heretofore been proposed a channeled pipe having a number of air channels continuously axially extending through the wall thereof as a way of saving raw material and increasing the flattening rigidity of the pipe. Particularly, channeled plastic pipes, such as are made of rigid polyvinyl chloride, can be compared with ordinary, non-channeled plastic pipes with regard to the surface conditions and basic physical properties, e.g., tensile strength. Thus, the technique for production of channeled plastic pipes has been established.
Because of their high flattening rigidity, such channeled plastic pipes are utilized as buried sewage pipes, in which case the manner of connecting them together is, e.g., as illustrated in FIG. 1. The left-hand end of each pipe 1 is a socket end 1a increased in diameter by being subjected to secondary processing and the right-hand end is a spigot end 1b not subjected to secondary processing. Thus, these pipes are of the so-called socket-spigot type. Therefore, a continuous line of pipe can be formed by inserting the spigot end 1b of each pipe 1 into the socket end 1a of the adjacent pipe 1 at its right end, and is buried in the ground 2. FIG. 2 is an enlarged section showing details of the socket-spigot joint. Air channels 1d extend continuously axially through the pipe wall. In addition, in the view taken along line III--III shown in FIGS. 3(A) and (B), the former refers to an oval (in cross-section) pipe 1A and the latter to a round (in cross-section) pipe 1B. In each case, a number of air channels 1d are formed in the pipe wall. Besides these cross-sectional shapes, there are many others, each presenting similar problems. Thus, if the pipeline is buried in the ground in the illustrated condition, rainwater, underground water or the like may flow in, as indicated by an arrow P, through the air channels 1d and out at the spigot end, as indicated by an arrow Q, into the pipeline or the water flowing through the pipeline may leak out through the air channels in the reverse direction. In such channeled plastic pipes, it becomes necessary to close the air channels 1d at the socket or spigot ends thereof. In addition, the socket end shown in FIGS. 1 and 2 has a first diameter-increased portion 1c and a second diameter-increased portion 1e, the latter having a plastic or metal retainer ring 3 set in the inner surface thereof for retaining a packing member 4 having an annular groove shaped complementary to the exposed portion of the retainer ring 3. The formation of such air channels is not limited to the pipe having the socket end shown in FIG. 2. Such air channels are often formed in various pipe constructions, including a simple extruded pipe with no secondary processing applied to either end thereof and a socket-spigot pipe with a single diameter-increased portion having a packing receiving annular groove 5 as shown in FIG. 4. Of course, there are also similar problems with these embodiments.
Therefore, it has been thought necessary to close such air channels at either the socket end or the spigot end thereof to prevent entry of outside water or leakage of inside water. For example, European Patent Publication Nos. 13,457 and 13,458 disclose a mechanism for applying a force which tends to squeeze or expand the pipe wall to thereby press the outer wall surface against the inner wall surface or the latter against the former, or force-fitting a blind plate to the pipe end surface. The portion thus processed, however, is formed with a constriction, which is a structural defect, on the outer or inner surface thereof, possibly leading to damage to the pipe unless special measures for reinforcement are taken. Further, since the pipe wall is only crimped under pressure, the resulting end-closing effect is insufficient, requiring the application of an auxiliary part in the form of a blind plate. However, such blind plate applied to the pipe end cannot be said to be sufficient since it could be easily removed by external forces unless it is held under adequate pressure.
Japanese Patent Laid Open Nos. 52222/1977, 127481/1979, 37344/1980, 73514/1980, 10681/1979, and 103480/1979 disclose means for crimping the end edge or end portion of pipe over a given length; however, the sealing quality of the crimped portion cannot always be said to be satisfactory, and water could pass therethrough. Further, Japanese Patent Laid Open No. 73512/1980 and Japanese Utility Model Laid Open Nos. 83024/1979 and 114812/1979 disclose means for fitting a plug into the end of each air channel, however, it is very difficult to ensure satisfactory positioning between the air channels and plugs. Another drawback is that the plugs tend to come off the air channels when subjected to a high outward force produced as the water pressure in the pipe builds up. Further, Japanese Patent Laid Open No. 123180/1979 discloses a method of closing air channels by injecting a foamable material into the air channels and then causing it to foam. However, it is very troublesome to inject such material individually into the air channels. Since it is difficult to uniformly control the amount of injection and the degree of foaming, the sealing quality differs in different air channels and some air channels are insufficiently closed while others are overfilled with such material, detracting from over all economy. There have been many other suggestions but none has been known which is satisfactory from the standpoint of easiness of operation, reliability and uniformity of closure, economy, external appearance, etc.